Transpressional tectonics and Carboniferous magmatism in the Limousin, Massif Central, France: Structural and ⁴⁰Ar/³⁹Ar investigations

New structural, microstructural, and 40Ar/39 Ar data from the NW Massif Central (France) provide additional constraints on the timing and tectonic setting of late Variscan granite magmatism. Previous studies had emphasized the role of late orogenic extension in the emplacement of granite plutons in the Limousin region. In contrast, the new data set is consistent with syntectonic emplacement of magma in a dextral simple shear active from 350 to 300 Ma in a transpressional regime. As an alternative hypothesis to late orogenic extension, we propose that magmas migrated into tensional bridges between active P shears associated with a lithospheric shear zone comparable to a pop-up structure. The Galician region, in the western end of the Ibero-Armorican tectonic arc, exhibits major left-lateral ductile shear zones which can be interpreted as conjugate structures to the Limousin and Armorican shear zones. Copyright 2007 by the American Geophysical Union

The Vi antigen of Salmonella typhi

Salmonella typhi, l'agent étiologique de la fièvre typhoïde humaine, exprime un polyoside capsulaire appelé antigène Vi. Certains auteurs ont impliqué cet antigène dans la virulence de s. typhi, plus particulièrement dans la résistance à l'activité bactéricide du sérum humain normal. Cependant, S. typhi étant une bactérie strictement adaptée à l'homme, le rôle de l'antigène Vi dans la virulence de cette bactérie reste controversé faute de modèle expérimental animal. Récemment, pour une meilleure appréhension du rôle de cet antigène dans la virulence, l'étude des mécanismes impliqués dans la biogenèse de l'antigène Vi a été entreprise. Il apparaît que l'expression de cet antigène est régulée par au moins deux systèmes de régulation à deux composants, les systèmes OmpR-EnvZ et RcsB-RcsC, ce qui suggère un rôle important du micro-environnement rencontré par S. typhi dans l'expression de l'antigène Vi, et par là même, de sa virulence. Par ailleurs, les propriétés immunogéniques et protectrices de l'antigène Vi ont été utilisées à des fins vaccinales. Un vaccin polyosidique, constitué exclusivement d'antigène Vi, a été mis au point. La protection observée dans des régions où la fièvre typhoïde est endémique, est supérieure à 65 % et, trois ans après la vaccination, les titres d'anticorps anti-Vi restent très élevés. Le développement de vaccins constitués d'antigène Vi conjugués à des protéines porteuses est en cours, dans le but, notamment, d'obtenir l'augmentation des taux de protection contre la fièvre typhoïde chez les enfants

Carving mountains : aeolian erosion as a major factor controlling the topographic evolution in the Gobi region (Mongolia).

International audienceThe Tsagaan Els Basin in SE Mongolia is a 31 000 km2 endoreic depression submitted to active erosion and surface lowering. The present-day depression is superimposed to a complex pattern of Cretaceous grabens and semi-grabens separated by basement highs composed of Proterozoic to Paleozoic metamorphic and magmatic rocks (Johnson et al., 2001). The summits of those basement highs correspond to fragments of a very low relief planation surface that formed prior to the Cretaceous extension phase, most probably during the Early to middle Jurassic (Jolivet et al., 2007). The Late Cretaceous sediments largely seal the extension phase. Paleogene to Neogene sedimentation is extremely condensed, marked by numerous aeolian dune deposits and carbonated paleosols, both indicating a long-lasting arid to semi-arid climate. The Quaternary sediments are limited, mainly composed of aeolian dunes, thin alluvial deposits, patchy outcrops of shale corresponding to small ephemeral lakes, and grain-thick sheets of gravels to pebbles covering strongly winnowed surfaces. Mapping of the drainage system shows a first-order trend towards the lowest point of the basin, associated to a second-order highly complex river network associated to local base levels. Numerous river capture events are observed indicating a poorly stabilized, locally constrained drainage system. In the center of the Tsagaan Els Basin, the c.a. 55 km long, 20 km wide Dulan Ul hill corresponds to one of the basement horsts, cropping out of the Cretaceous series. This flat-topped mountain, culminating around 950 m a.s.l. and 200 m above the basin is bordered to the north by a large fan-like morphological structure carved within the Cretaceous series. Similar fan-shaped morphologies are observed on several locations within the Tsagaan Els Basin, that cannot be explained through alluvial deposits or river incisions.In this study, we performed a detailed mapping of the displacement of the sand (that largely corresponds to the mode of wind direction) over the central part of the Tsagaan Els Basin. We show that the fan-like morphology upwind of the Dulan Ul hill is effectively related to wind-erosion carving into the Cretaceous sediments in front of the basement high. This strong aeolian erosion, associated to the occurrence of large lithological contrasts between the basement and the sedimentary series explains the complex and extremely versatile drainage network